The present invention relates generally to photochromic products and articles, i.e., products or articles comprising a photochromic substance capable of reversibly changing color or opacity when exposed to predetermined radiation, and more particularly, to a photochromic substrate such as an ophthalmic lens of organic material, and a process of integrating a photochromic substance into such a substrate or lens.
Photochromic ophthalmic lenses which have been commercialized up to the present, for sunglasses, are generally of mineral glass, and the photochromic substance which they comprise is also of a mineral nature. Most frequently, the substance is a silver halide.
Tests have been performed with a view to using such a photochromic substance in lenses of organic material; the conclusion reached was that there is in practice incompatibility between the two, as the silver halide in the midst of the organic material does not have a suitable reaction to external radiation.
As for organic lens sold under the trade designation CR39 and consisting of poly [ethylene glycol di-allyl di-carbonate] this incompatibility is due to the nature of the catalyst required for the polymerization of the basic monomer. The only catalyst now in use which gives satisfaction for such a polymerization is isopropyl percarbonate, which by reason of oxidation destroys the photochromic properties of the silver halide.
To overcome this difficulty it has been attempted to coat the silver halide particles with an inorganic protective coating, of silica for example, which is impermeable to organic products; this is a costly solution which is moreover inefficacious.
It was then attempted to substitute the silver halide with a photochromic substance of organic nature. Various proposals have been made in this regard whereby such a substance is dispersed throughout the body of the organic material constituting the lens. No such proposal has proved really satisfactory.
One of the reasons that such proposals have not been satisfactory lies, as above, in the incompatibility of the photochromic substances with the polymerization catalyst, and in the practical difficulties encountered in performing such procedures, namely the coating, which was envisaged to obviate this problem.
Other proposals have consisted in consolidating the photochromic substance in the midst of a particular layer intermediate two outer layers of organic material; performance of such a method is tedious.
Yet further proposals treat otherwise finished lenses, and therefore after polymerization of their constituent organic material, so as to avoid contact between the photochromic substance utilized and the catalyst for the polymerization.
According to a first of these last proposals, the photochromic substance is deposited in a film on the ophthalmic lens to be treated, such a film is inevitably subjected to impacts, scratching and abrasion thereby spoiling its continuity and therefore the homogeneity of the sought after photochromism.
In a second of these last proposals, the ophthalmic lens to be treated is immersed in a concentrated solution of the photochromic substance in a solvent, in practice toluene, at ebullition, 70.degree. C.; the photochromic substance may be spiropyran as disclosed in French printed patent application No. 2,211,666 or a metal dithizonate as disclosed in French printed patent application No. 2,236,479.
However, in order to sufficiently impregnate the ophthalmic lenses treated, the immersion must be prolonged, for at least 24 hours, and on account of the boiling of the solution used, special procedures were required to satisfactorily recover the solvent vapors, thereby making such a process rather difficult to perform. Moreover, the results obtained were disappointing.
Furthermore, organic photochromic substances known to date are subject to ageing; after a rather short time, in any event less than the normal life of sunglasses, they become inert and no longer react to the radiation to which they are exposed.
Considering it is a substance dispersed deep in the ophthalmic lens concerned, such ageing inevitably means that the sunglasses have become worthless.
If such a substance is deposited in a film on the ophthalmic lenses it would obviously be conceivable to regenerate the same once it has aged, but in practice, for reasons already developed, such a regeneration with processes known to date, has proven to be inexpedient and too expensive to be worth while. Accordingly such lenses too would become worthless after ageing.